The indoor growing of plants may be preferred to outdoor methods when space is limited, the length of the growing season is too short or even when environmental conditions are unsuitable. This may also allow the opportunity to limit exposure to pests and diseases. However, these advantages come with a significant added cost of providing sufficient light intensity and the required energy. Of the commonly available lighting technologies, light emitting diodes (LEDs) are the most efficient, but the spectrum of energy versus wavelength is often incorrect for optimal plant health and growth rate.
Recently, new phosphors have been developed for the efficient conversion of blue LED light into longer wavelengths with a better match to plant requirements, having significant emission beyond 600 nm. However, it is not possible to adjust the spectrum of these lights throughout the day and over the growing season. In addition, there are benefits to the use of wavelengths both above and below the wavelengths which are visible to the human eye, for which there are no efficient phosphors. Several lighting systems have been developed using multiple LED types, which allow different wavelengths to be combined, but these systems do not incorporate the full spectrum of usable wavelengths, nor do they allow flexible control over spectral intensity throughout the day and over the growing season.
As shown in FIG. 1, several important light absorbing chemicals may be stimulated using different wavelengths of light. The chemicals most responsible for converting light energy in the chemical energy to fuel growth are chlorophyll A and B which may be present in different amounts, depending on the plant species and growing conditions. The energy production process may also be assisted by accessory pigments such as carotenoids and phycocyanins. The light absorption of chlorophyll-a is centered around about 430 and about 660 nm, but chlorophyll-b in centered around about 460 and about 650 nm. By providing two channels of blue light at about 430 and about 460 nm the grower can tune the spectrum to the ratio of A to B chlorophyll absorption required. While aquatic plants and corals can grow well with little to no input of light near about 660 nm, these organisms have adapted to an environment which is low in red light due to absorption by the water column above. Terrestrial pants are known to prosper in light which is stronger in red than in blue wavelengths, which is often accomplished by the addition of light near about 660 nm. This is also able to stimulate both chlorophylls and improves the production of energy, increasing plant yield.
It was shown first by Emerson, in 1957 [Emerson, Robert (1957). “Dependence of yield of photosynthesis in long wave red on wavelength and intensity of supplementary light”. Science 125: 746] that the exposure of plants to wavelengths beyond about 700 nm, which were not thought at the time to be involved in photosynthesis, resulted in a significant increase in the rate of energy production. It was later discovered that the chemical phytochrome absorbs light near about 660 nm and undergoes a chemical transformation to a form that absorbs light near about 730 nm, and upon the absorption of light near about 730 nm, this transformation is reversed [Walker T S, Bailey J L. Two spectrally different forms of the phytochrome chromophore extracted from etiolated oat seedlings. Biochem J. 1968 April; 107(4):603-605]. The relative concentrations of the red-absorbing (PR) and Far-red absorbing (PFR) forms acts as a signaling mechanism which can inform the plant as to the time of dusk and dawn and to the length of the daylight period. This can in turn be used not only to increase photosynthetic rates, but also to control the growth habit of plants or to control their flowering behavior [Gururani, Mayank Anand, Markkandan Ganesan, and Pill-Soon Song. “Photo-biotechnology as a tool to improve agronomic traits in crops.” Biotechnology Advances (2014)].